Delivering the correct dose to the intended area is the most basic goal of any radiotherapy treatment. The most direct way to confirm the true dose delivered to a location of interest is to measure that dose in situ. In prostate radiotherapy, the outcome (biochemical and local control) depends on accurate delivery of the dose to the prostate as planned. At the same time, it is important to ensure that the tolerance to critical structures (rectum, urethra, and erectile tissues) is within acceptable limits so that toxicity is minimized and quality of life is not compromised. We hypothesize that dose-based adaptive radiotherapy is achievable by measuring the dose delivered to organs, critical structures, and within the vicinity of the tumor. We believe this can be done using an in vivo scintillation detector composed of multiple probes arranged in an application-specific design that can monitor true in vivo dose in real time. Plastic scintillation detectors are constructed from three main components: a miniature scintillating material that luminesces (emits visible light) when irradiated, an optical guide that carries the light, and a photodetector that converts the light into a measurable signal. We will use sub-millimeter diameter scintillating fibers for their spatial resolution as well as their flexibility, allowing them to conform to the curvatures of internal anatomy. In order to reach this goal, we aim to: a) establish the dosimetric characteristics and properties of scintillating fibers in photon and proton radiotherapy beams, b) design, construct, and test detector systems for rectal and urethral in vivo applications, and c) measure the dose to the rectal wall and urethra for a small cohort of patients. Successful completion of this project will result in a method for monitoring the true dose delivered to organs and other tissues at risk during radiotherapy. This method can be used to generate data to assess the dose to the organs and critical structures and alter the treatment plan to maximize the dose to the tumor and/or minimize the risk of complications to normal tissue. The resulting data can also be used to study dose- related treatment side effects. The ultimate goal of utilizing this method is to improve the delivery of radiotherapy treatments and the quality of life of radiotherapy patients.